1. Field of the Invention
The present disclosure relates to contrast imaging. More particularly, the present disclosure relates to apparatus and methods for contrast imaging in an ultrasound device in which elevation beamwidth is controlled to reduce image artifacts, which result from a contrast agent being carried into the image region by motion or respiration and not by reperfusion.
2. Description of the Related Art
Ultrasound imaging is widely used for medical imaging due to its safety, good image quality, and low cost and includes both tissue imaging and contrast imaging. Contrast imaging involves the use of intravenous contrast agents, which are encapsulated micro-bubbles, which are prepared and injected intravenously into the venous return system of the body of a patient. In particular, micro-bubbles are bubbles in the range of less than about 15 microns in diameter and include a heavy gas or air encapsulated with some form of shell, which can be, for example human albumin, a lipid protein, or a sugar. Micro-bubbles have characteristics that make them useful in contrast imaging both as tracers to show blood flow to or through the imaged tissue and also to quantify the rate of blood flow to or through the imaged tissue. The contrast agents can be easily detected and imaged because they resonate in the presence of ultrasonic fields producing radial oscillations. This resonant behavior gives rise to a non-linear acoustic response, typically a second harmonic response; but there are also other non-linear responses that can be detected.
A second characteristic of micro-bubbles, which is also useful, is that if excited with a large enough acoustic pressure in the range of contemporary ultrasound systems, the micro-bubbles can be destroyed by disrupting their shells and thereby allowing the encapsulated gas to escape. The gas from the destroyed micro-bubbles then dissolves into the blood stream.
Several techniques have been developed which take advantage of the primarily linear response behavior of tissue to cancel or attenuate tissue signals when ultrasound imaging is applied to the body of a patient, thereby providing an improved image of the contrast agents. In several of these techniques, multiple transmit lines are fired along the same line of sight into the body of the patient. The transmit waveform is modified (e.g., in terms of power, phase, or polarity) from line to line to produce a variation in the response received by the ultrasound transducer. These data points are then processed to remove the influence of their linear components, which are primarily related to the tissue signal, to yield data that primarily contains the non-linear response of the contrast agents.
Regardless of the technique used to image contrast agent, it is desirable to determine the rate of flow (perfusion) of blood in myocardial tissue. Because the flow rates are low, a general technique has been employed to measure the rate of reperfusion as follows. The contrast agent is administered to the patient, after which the contrast agent enters the right side of the heart and myocardial tissue. The contrast agent is first imaged and then is destroyed by high intensity ultrasound signals, thereby creating a ‘negative bolus’ effect. After the contrast agent is destroyed, the area of interest is subsequently scanned (imaged) and the intensity due to contrast agent is observed as it increases, indicating reperfusion within the scanned tissues.
Although the above-mentioned techniques work well in removing the influence of the tissue signal, artifacts from the contrast agent, which does not have a primarily linear response behavior, moving into the image plane caused by motion can degrade the resultant images. In particular, to obtain an accurate estimate of the rate of blood flow, only reperfusing contrast agent reperfusion, which corresponds with the rate of blood flow, should be imaged. The above-mentioned techniques are used to cancel or attenuate the linear signals of the tissue, and can include cancellation or attenuation of linear signals from tissue, which moves by either patient motion or respiration into the image area. These techniques, however, do not effectively compensate for the effect of the micro-bubbles in the moving tissue because the micro-bubbles produce this primarily non-linear response. Contrast agent, lying outside of the area, which was depleted of the contrast agent by high intensity ultrasound, can be carried by motion into the imaging plane. The contrast agent movement into the image plane can cause degraded images that cannot be effectively compensated for with typical processing techniques. This degradation can be substantial, particularly where the heart or lungs is being imaged due to their frequent and rapid motions.
What is needed is an apparatus and method for contrast imaging in which the response of a contrast agent from the area surrounding the image plane, which moves into the image plane, is effectively suppressed so as to enhance the imaging of only reperfused contrast agent, thereby increasing the accuracy of the imaging of the rate of the blood flow to or through the tissue in the image plane.